Perhaps the most pervasive problem faced by listeners with hearing impairment or cochlear implants is the difficulty of recognizing speech and other sounds in the presence of competing sound sources, as when conversing at a restaurant. This difficulty in ?sound segregation? ? hearing a particular sound of interest when it is embedded in a mixture of other sounds ? often leads to frustration and social isolation, and is not adequately addressed by current hearing aids and implants. Sound segregation difficulties are also commonly reported in developmental auditory disorders. The long-term goal of the proposed research is to reveal the basis of sound segregation and to provide insights that will facilitate improved prosthetic devices and remediation strategies, as well as more effective machine systems for processing sounds, e.g. for automatic speech recognition. The development of more effective devices, technologies, and therapies is currently limited by an incomplete understanding of the factors that underlie sound segregation by normal-hearing listeners. In particular, little is known about sound segregation with complex naturalistic sounds, in part because much of the research in this area has been conducted using simple signals that are impoverished relative to the sounds listeners normally encounter. We propose to enrich the understanding of sound segregation with three sets of experiments that use novel sound synthesis methods to manipulate properties of natural speech and other sounds and test their role in segregation with behavioral experiments in human listeners. Aim 1 manipulates the classically proposed grouping cue provided by harmonic frequency relations and investigates the mechanisms underlying their effect. Aim 2 investigates the role of prior knowledge of voice and speech structure on segregation, and should help to explain why some voices are easier or harder to segregate than others. Aim 3 investigates the role of attentive tracking in the segregation of sounds from mixtures, and will explore the factors that facilitate tracking or cause it to fail. The results will reveal the mechanisms underlying sound segregation by the healthy auditory system, and will provide insights into the factors that limit auditory comprehension in the presence of multiple sound sources, hopefully suggesting new strategies for signal enhancement, prosthetic devices, and behavioral remediation.